Hello, I am Mapharazzo from EUNE, currently a diamond 5 player. I enjoy playing Kha'Zix the most, and after many games played with him, I decided to make a guide which reveals all of his aspects.

Pros:
-high damage to single targets;
-awesome escape mechanics;
-easy roaming;
-fun to play;
-cool skin.

Cons:
-squishy if not played right;
-needs to enter in a teamfight at the right time;
-can fall off behind;
-hard to master (this can be a pro too).

In my opinion, he is extremely viable in ranked play, still now, due to his high surviability and killing potential. When played right, he has very very few counters, such as Lissandra and Ahri.

I choose the default runes for an AD top, because Kha needs to be able to dive and AA even once on an enemy champion in a teamfight.

The 21/9/0 is my best option yet, due to high durability start.

You might wonder: "Why Trinity Force?" Well, i think that with it's recent buff, it has become a must have item for AD casters. The Muramana is quite overrated on Kha'Zix builds, since it stacks way too hard, completed only at 30 minute mark by a not fed kha.
What boots to get:
-few enemy CCs --- Ionian;
-hard CCs/2 APs --- Mercury

At this point, I suggest evolving Q,E,R because W has no damage bonus at all, and the utility is really bad compared to other choices.
You have alternatives:
1. Versus a heavy harass lane: max W for the heal, but evolve Q.
2. Versus a squishy assassin: max and evolve Q.

Kha'Zix has a unique spell kit, he can deal massive damage within a short period of time.
To burst a target, you should always use this combo:
W ---> E ---> Q ---> AA ---> Q
Why? Because you can't use W while flying anymore and you need to use it efficiently, so this is the best option. Moreover, ALWAYS use Q WHILE FLYING. This increases your burst amazingly.

Don't be afraid to use your Q to take minions, it has greater range and damage than your autoattacks and it doesn't cost a lot of mana.